This invention relates to an electron beam system and particularly to an electron beam projection system for an electron beam exposure system.
In an electron beam exposure system for projecting to a sample a circular Gaussian beam, namely, a crossover image, image formation is made with large-diameter beams when no precision is required therefor and is made with small-diameter beams when high precision is required therefor. Further, it is demanded that a desired beam diameter is obtained by changing the diameter of a beam in units of, for example, 0.1 .mu.m with the brightness thereof kept fixed, thereby to increase the yield of masks on which images are formed. Further, it is also demanded that the beam diameter, beam current and beam shape (for example, the degree of beam circle) are precisely controlled for the purpose of increasing the precision of the image pattern formed on the sample.
In order to give a better understanding of the electron beam system we will hereinbelow explain the method of controlling the beam current, beam diameter, and beam shape which is obtained for a conventional electron beam system, by reference to FIG. 1. In the FIGURE, 101 denotes an electron gun, 102 an axis-alignment device for the electron beam from the electron gun 101, 103 a condenser lens for converging the electron beam, 104 an axis-alignment device for the electron beam which has passed through the condenser lens 103, 105 an astigmatism correction lens for correcting the astigmatism of the lens, 106 an objective lens, and 107 an electron beam detection device. A sample 108 onto which an electron beam is irradiated is arranged at the back stage of the electron beam detection device 107. In the electron beam detection device 107 detection is made of the beam current, beam diameter and beam shape. When each of the above-mentioned three controlling quantities does not reach a predetermined value, the feedback quantities which correspond to the detected value of the three controlling quantities are fed back to the above-mentioned six controlling devices 101 to 106. In this controlling method, however, since the feedback loops are provided, for example, six in number as shown, operational interference may occur between each loop. Assume now that the brightness of a beam is found not to have reached a desired value as a result of having detected, for example, the beam current and beam diameter, by the electron beam detection device 107. The brightness of a beam is given as in the following equation (1) from the diameter and current of the beam. EQU B=AI/d.sup.2 ( 1)
where I represents the beam current, d the beam diameter, and A a coefficient peculiar to this type of electron beam system. The brightness B of a beam is determined in accordance with the operational state of the electron gun 101, for example, the heating current of a cathode of the electron gun and/or the bias voltage applied to a Wehnelt electrode thereof. Accordingly, the beam brightness is controlled by controlling the electron gun 101 in accordance with the quantities detected by the electron beam detection device 107. However, when the operation state of the electron gun 101 is varied, the state of the electron beam such as the current, diameter and shape thereof is varied. For this reason, in order to measure the reset values of the beam current and beam diameter by means of the detection device 107 it is necessary to adjust the electron gun 101, axis-alignment device 102, lens axis-alignment device 104, astigmatism correction lens 105 and objective lens 106, respectively, so that they are brought to optimum states. However, for the reason that the feedback loops are provided large in number as shown it takes a long time to obtain desired values of the controlling quantities such as the beam diameter and beam shape. Further, at the current level of technique, the measuring precision of the beam diameter and the reproducibility of the measured values are respectively 0.1 .mu.m. Therefore, the total precision is approximately 0.2 .mu.m. The degree of beam circle is approximately 0.1 .mu.m in terms of the total precision. Accordingly, the percentage error in measuring the brightness of a circular beam of 0.5 .mu.m diameter frequently becomes as great a value as over 100% in consideration of the reproducibility of that brightness. Controlling by the same or common controlling means both the beam current and the beam diameter together as in the case of the conventional electron beam system results in a decrease in the measuring precision of the brightness down to a value less than mentioned above. Such controlling, therefore, almost fails to obtain the desired brightness of a beam.